Energy storage system comprising a modular multi-level converter

Abstract

An energy storage system (ESS) for an electrical system includes an energy storage, and a converter interface arranged for connecting the energy storage to the electrical system. The converter interface includes a modular multilevel converter in which each phase leg includes a plurality of series connected cells of which at least one is a half-bridge cell and at least one is a full-bridge cell.

The invention claimed is: 
     
       1. A method performed in an energy storage system (ESS) being connected to a high voltage AC electrical system, the ESS comprising an energy storage and a converter interface having a DC link and connecting the energy storage to the high voltage AC electrical system, wherein the converter interface comprises a modular multilevel converter in which each phase leg comprises an upper arm connected to a positive pole of the DC link of the modular multilevel converter and a lower arm connected to a negative pole of the DC link of the modular multilevel converter, each arm comprising a plurality of series connected cells of which at least one is a half-bridge cell and at least one is a full-bridge cell, the method comprising:
 when a DC link voltage (VDC, batt) provided by the energy storage is more than or equal to a peak-to-peak voltage (VAC, peak-peak) of the high voltage AC electrical system, operating the at least one full-bridge cell to produce positive voltage; and subsequently 
 when the DC link voltage (VDC, batt) provided by the energy storage is less than the peak-to-peak voltage (VAC, peak-peak) of the high voltage AC electrical system, operating the at least one full-bridge cell to produce negative voltage. 
 
     
     
       2. The method of  claim 1 , wherein the ESS has a nominal voltage of at least 30 kV or at least 50 kV, and/or up to 70 kV or up to 100 kV. 
     
     
       3. The method of  claim 1 , wherein the modular multilevel converter has three, and only three, phase legs. 
     
     
       4. The method of  claim 3 , wherein the ESS has a nominal voltage of at least 30 kV or at least 50 kV, and/or up to 70 kV or up to 100 kV. 
     
     
       5. The method of  claim 1 , wherein the energy storage comprises an electrochemical battery or a supercapacitor. 
     
     
       6. The method of  claim 5 , wherein the modular multilevel converter has three, and only three, phase legs. 
     
     
       7. The method of  claim 5 , wherein the ESS has a nominal voltage of at least 30 kV or at least 50 kV, and/or up to 70 kV or up to 100 kV.